1. Field of the Invention
The present invention relates to an input-output interface circuit structure installed between an input-output processing device and an external input-output terminal, which transmits an external input signal from the external input-output terminal to the input-output processing device as an input signal and also transmits an output signal from the input-output processing device to the external input-output terminal as an external output signal.
2. Description of Related Art
Computer systems equipped with central processing units (which are hereafter referred to as CPUs as necessary) typically incorporate a plurality of external input-output terminals for the central processing unit. Such a computer system has a signal processing means or device to process signals input to and output from the central processing unit, and an input-output interface circuit between the signal processing device and the external input-output terminals. The signal processing device, which is governed in operation by the central processing unit, performs specific processing on signals input thereto to convert them into input data to the central processing unit, and performs specific processing on output data coming out of the central processing unit to convert them into output signals. The input-output interface circuit transmits external signals from the external input-output terminals to the signal processing device as input signals and also transmits signals output from the signal processing device to the external input-output terminals as external signals.
This type of input-output interface circuit utilized in a computer system is well known in the art and shown by way of example in FIG. 4. An signal processing circuit, shown as being accompanied by an external input-output terminal 11, is comprised by an input signal forming network consisting of serial connection of an input buffer circuit 12 and a latch circuit 13, through which an external signal (SIP) input from the external input-output terminal 11 is transmitted to the signal processing device, and an output signal forming network consisting of serial connection of a latch circuit 14 and an output buffer circuit 15, through which an output signal (SOP) from the signal processing device is transmitted to the external input-output terminal 11. The input buffer circuit 12 and output buffer circuit 15 are comprised, for example, of a tri-state type of buffer amplifiers which assume ON and OFF states according to externally supplied control signals (CSP): the input buffer circuit 12 going ON when the output buffer circuit 15 goes OFF; and the input buffer circuit 12 going OFF when the output buffer circuit 15 goes ON. While an externally supplied control signal (CSP) turns the input buffer circuit 12 OFF and the output buffer circuit 15 ON, an external signal (SIP) is supplied, as an input signal to be input to the signal processing device, to an output side of the latch circuit 13 through the input signal forming network, i.e. the serial connection of the input buffer circuit 12 and latch circuit 13, from the external input-output terminal 11. On the other hand, while a control signal (CSP) turns the input buffer circuit 12 ON and the output buffer circuit 15 OFF, an output signal (SOP) from the signal processing device runs through the output signal forming network, i.e. the serial connection of the latch circuit 14 and output buffer circuit 15, and is transmitted as an external output signal to the external input-output terminal 11 after amplification at the output buffer circuit 15.
FIG. 5 shows another example of a conventional input-output interface circuit accompanied by an external input-output terminal means or device 11, which incorporates multiplexers 16 and 17 installed on opposite sides of a latch circuit 18, respectively, in addition to an input buffer circuit 12 and an output buffer circuit 15 similar to those incorporated in the conventional input-output interface circuit previously described and shown in FIG. 4.
The multiplexer 16 is equipped with two input terminals and one output terminal. An output signal (SOP) is fed to one of the input terminals of the multiplexer 16 from the signal processing device, and an input signal (SIP) is fed to another input terminal of the multiplexer 16 from the external input-output terminal device 11 through the input buffer circuit 12. While the output buffer circuit 15 goes OFF with an external control signal (CSP), the multiplexer 16 responds to the external control signal (CSP) by accepting, at its output terminal, an external input signal (SIP) from the external input-output terminal device 11 through the input buffer circuit 12. On the other hand, while the output buffer circuit 15 goes ON with an external control signal (CSP), the multiplexer 16 also responds to the external control signal (CSP) by accepting an output signal (SOP) from the signal processing device . Similarly, the multiplexer 17 is equipped with two output terminals and one input terminal, and receives at the input terminal either one of the output signal (SOP) and the external input signal (SIP) through the multiplexer 16 via the latch circuit 18. While an external control signal (CSP) puts the output buffer circuit 15 into an OFF state, the multiplexer 17 responds to the external control signal (CSP) by transmitting the external input signal (SIP) to either one of the output terminal; and while an external control signal (CSP) puts the output buffer circuit 15 into an ON state, the multiplexer 17 responds to the external control signal (CSP) by transmitting an output signal (SOP) to another output terminal.
In addition, while an external control signal (CSP) puts the input buffer circuit 12 in the ON state and the output buffer circuit 15 in the OFF state, the input signal (SIP) from the external input-output terminal device 11 passes through the input buffer circuit 12 and enters the multiplexer 16 through one of the input terminals and then to the latch circuit 18 through the output terminal of the multiplexer 16. It is further transmitted to the signal processing device as an input signal from the multiplexer 17 through one of the output terminals. Further, while an external control signal (CSP) puts the input buffer circuit 12 in the OFF state and the output buffer circuit 15 in the ON state, an output signal (SOP) from the signal processing device is transmitted to one of the input terminals of the multiplexer 16 and then to the output buffer circuit 15 through the multiplexer 17 via the latch circuit 18. The output signal (SOP) is amplified at the output buffer circuit 15 and finally transmitted to the external input-output terminal 11 as an external signal.
As previously stated, in the case where computer system incorporates a signal processing device installed between a central processing unit and multiple external input-output terminals so as to process signals input to and output from the central processing unit, and an input-output interface circuit connecting the signal processing device the external input-output terminals, from the point of view of monitoring wave forms of signals input to and output from the signal processing device, there is a strong demand on the input-output interface circuit for an operating mode in which an signal output from the signal processing device is re-input to the signal processing device again as an internal signal as well as an operating mode in which an external signal is input to the signal processing device as an input signal from the external input-output terminal and an operating mode in which a signal output from the signal processing device is supplied to the external input-output terminal as an external signal.
The prior art input-output interface circuit shown in FIG. 4 or 5 has no signal transmission channel which allows a signal output from the signal processing device to return to the same again as an internal signal and consequently, is incapable of re-directing the output signal from the signal processing device toward the signal processing device without first transmitting it to the external input-output terminal. In other words, in order for the prior art input-output interface circuit to re-direct the output signal from the signal processing device toward the signal processing device as an internal signal, a special internal signal input port must be installed for internally transmitting the internal signal to the signal processing device.